Cholesteric polarizer and the manufacture thereof

ABSTRACT

Cholesteric polarizer and the manufacture thereof. A broadband cholesteric polarizer is described, as well as a method of manufacturing such a polarizer. Said polarizer comprises an optically active layer of a polymer material having a cholesteric order, said material being oriented so that the axis of the molecular helix extends transversely to the layer. In accordance with the invention, the polarizer is characterized in that the pitch of the molecular helix in the layer is varied in such a manner that the difference between the maximum pitch and the minimum pitch is at least 100 nm. The optically active layers are preferably provided on substrates which bring about a conversion of circularly polarized light into linearly polarized light. Three different methods of manufacturing such broadband polarizers are described.

This is a continuation of application Ser. No. 08/179,420, filed Jan.10, 1994, now U.S. Pat. No. 5,506,704.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cholesteric polarizer comprising an opticallyactive layer of a polymer material having a cholesteric order, thematerial being oriented in such a manner that the axis of the molecularhelix extends transversely to the layer. The invention also relates tomethods of manufacturing such polarizers. The invention further relatesto a lighting device comprising a socket for an electric light source, areflector and such a cholesteric polarizer which is preferably providedwith a quarter-wave plate.

2. Discussion of the Related Art

Polarizers are used to convert unpolarized light into polarized light.Until now so-called "sheet polarizers" have been used for this purpose.When said sheet polarizers are exposed to unpolarized light theytransmit one of the two orthogonically linearly polarized components ofthe light, while the other component is absorbed in the polarizer. Suchpolarizers have the drawback that under optimum conditions maximallyonly 50% of the quantity of incident light is converted into polarizedlight. Thus, this type of polarizers has a relatively low efficiency.Another drawback relates to the absorption of the untransmittedcomponent. This may give rise to considerable heating of the polarizer,which causes undesired changes in the polarization characteristic of thepolarizer and, at high intensities of the incident light, can even leadto destruction of the polarizer.

By means of cholesteric polarizers it is possible to very efficientlyconvert unpolarized light into polarized light. Such polarizers comprisean optically active layer of a cholesteric (i.e. chiral nematic)material. In this type of liquid crystalline material the chiralmolecules have a structure such that they spontaneously assume aspiral-like or helical structure. After such a mixture has been providedas a thin, optically active layer between two parallel substrates, saidhelical structure is oriented in such a manner that the axis of thehelix extends transversely to the layer. A better orientation of thehelix is obtained if the substrates are provided with so-calledorientation layers on the surfaces facing each other. If this type ofpolarizer is irradiated with a beam of unpolarized light, the part ofthe light which is "compatible" with the (right-handed or left-handed)direction and pitch of the helix is reflected, while the remainder ofthe light is transmitted. By means of a mirror, the "compatible"polarization of the reflected light can be reversed, after which saidlight, which is now "incompatibly" polarized, can again be directed onto the polarizer. In this manner and using this type of polarizer,theoretically, 100% of the incident unpolarized light having a"compatible" wavelength can be converted into circularly polarizedlight.

Such a cholesteric polarizer is known from an article by Maurer et al.,entitled "Polarizing Color Filters Made From Cholesteric LC Silicones",from SID 90 Digests, 1990, pp. 110-113. In this article a description isgiven of cholesteric polarizers whose optically active layer consists ofa polymer material having a cholesteric order on the basis of silicones.This layer is manufactured by orienting a mixture of a chiral siliconemonomer and a nematogenic silicone monomer between two substrates ofglass, after which they are polymerized to the optically active layer bymeans of UV light. The ratio between the two types of monomer in thepolymer material governs the pitch of the molecular helix and thereflection wavelength (=colour of the reflection) associated therewith.The ratio between the pitch p and the wavelength λ is given by theformula λ=1/2.(n'+n")p, where n' and n" are the extraordinary and theordinary refractive index, respectively, of the polymer material.

An important drawback of the known cholesteric polarizer is that thebandwidth Δλ of the polarized light is much smaller than the bandwidthof the visible spectrum. This bandwidth is determined by the formulaΔλ=λ.Δn/n, where Δn=n'-n" represents the birefringence of the layer andn=(n'+n")/2 represents the average refractive index. The bandwidth inthe visible portion of the light spectrum is governed predominantly bythe birefringence of the cholesteric material. The possibilities ofincreasing said birefringence are relatively limited. In practice it hasbeen found that Δn is smaller than 0.3, so that the associated bandwidthis smaller than 100 nm. In general, the bandwidths have values rangingbetween 30 and 50 nm. This small bandwidth is problematic for manyapplications. In practice, polarizers having a bandwidth of at least 100nm, and preferably 150 nm and more are desired. In particular bandwidthswhich cover an important portion of the visible spectrum are veryinteresting for industrial applications.

In the above-mentioned article this known problem is overcome by the useof polarizers which are built up of a number of optically active layershaving different reflection wavelengths. In this manner, a polarizerhaving a bandwidth of 300 nm can be obtained which covers substantiallythe entire visible portion of the spectrum. However, this solution has anumber of important drawbacks. First, the optical quality of cholestericpolarizers consisting of more than one optically active layerdeteriorates rapidly due to errors which are typical of cholesterics.Said errors are, in particular, so-called "focal-conical" disclinations,"Grandjean"-disclinations and a loss of planar molecular order. Second,the thickness of such a composite polarizer causes problems. As thethickness of the individual layers must minimally be 6 microns, suchcomposite polarizers have a minimum thickness of approximately 20microns. At such thicknesses of the optical layer, the polarizer becomesexcessively dependent on the viewing angle.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a polarizer in which theabove drawbacks are overcome. The aim of the invention is, more inparticular, to provide a polarizer which is compact and of a simpleconstruction and which can be manufactured in a simple manner. Thebandwidth of the intended polarizer should be larger than that of theknown polarizers and should preferably comprise a substantial portion ofthe visible spectrum. The invention also aims at providing methods ofmanufacturing such polarizers in an efficient and cost-effective manneras well as lighting devices comprising such a polarizer.

These and other objects of the invention are achieved by a polarizer ofthe type mentioned in the opening paragraph, which is characterizedaccording to the invention in that the pitch of the molecular helix inthe layer is varied in such a manner that the difference between themaximum pitch and the minimum pitch is at least 100 nm.

In the known single cholesteric polarizers the pitch across theoptically active layer is substantially constant. As the pitch in theoptically active layer of the inventive polarizer varies, bandwidths ofat least 160 nm can be realised. As will be explained hereinafter,polarizers having a bandwidth in excess of 250 nm have been manufacturedwhich bandwidth covers substantially the entire visible portion of thelight spectrum (400-640 nm). This bandwidth is sufficient for manyapplications. As the inventive polarizer comprises only one opticallyactive layer, the above-mentioned drawbacks of the known multilayerpolarizer do not occur.

In accordance with a preferred embodiment of the invention, theinventive polarizer is characterized in that the pitch of the molecularhelix increases substantially continuously from a minimum value at onesurface of the layer to a maximum value at the other surface of thelayer. By means of this particular configuration it is attained that thehelical structure of the cholesteric material, viewed in the directionof the normal to the layer, changes gradually. This precludes theoccurrence of material stresses in the optical layer and has afavourable effect on the strength of said layer.

Another favourable embodiment of the polarizer according to theinvention is characterized in that the polymer material forms athree-dimensional network. Optically active layers which consist of sucha three-dimensional network are exceptionally strong. In practice theycan suitably be used as self-supporting polarization films. That is,said optically active layers need not be provided with substrates. Afterthe manufacture of such cholesteric polarizers the substrates which arenecessary for the orientation and polymerization can be removed. Thishas a favourable effect on the compactness of the polarizer. Further ithas been found that this particular type of polarizer has the additionaladvantage that the temperature dependence of the polarizationcharacteristic is extremely small.

A further interesting embodiment of the cholesteric polariser inaccordance with the invention is characterized in that the opticallyactive layer is present on a substrate of a stretched synthetic resinfilm, the degree of stretching and the thickness of the film beingselected in such a manner that the optical retardation of the film isapproximately 0.25 times the wavelength of the band reflected duringoperation of the polariser.

The light which is passed through a cholesteric polarizer is circularlypolarized. For a number of applications it is desirable that theemergent light is linearly polarized instead of circularly polarized. Inthat case, the polariser in accordance with the last-mentioned preferredembodiment is provided on a substrate of stretched synthetic resin film.Since the substrate is stretched in-plane in one direction, therefractive indices in said direction and in the in-plane directionperpendicularly to the direction of stretching are different. Therefractive index difference and the thickness of the film can beselected in such a manner that the product of these quantities (opticalretardation) corresponds substantially to 0.25 times the wavelength ofthe (centre of the) bandwidth reflected by the polariser. As a result,this substrate serves as a quarter-wave plate which converts circularlypolarized light into linearly polarized light. The synthetic resin filmmay be manufactured of, for example, polyethylene terephthalate,polycarbonate, polyethylene ketone or polypropylene.

If the inventive polariser is constructed as a self-supporting layer,said layer can be directly provided on such a substrate of stretchedsynthetic resin film, which substrate can serve as an additional supportfor the polariser. Since such a polariser does not have to be providedwith separate substrates, a compact "linear polariser" is obtained inthis manner. It is alternatively possible, however, that the stretchedsynthetic resin film already serves as a substrate in the manufacture ofthe optically active layer. This benefits the ease of manufacture.

A further interesting embodiment of the cholesteric polariser inaccordance with the invention is characterized in that the opticallyactive layer is present on a substrate of two stretched synthetic resinfilms of different composition, the directions of stretching of bothfilms extending substantially transversely to each other, and the degreeof stretching of both films being selected in such a manner that, due tothe difference in dispersion between the films, the net retardation ofthe substrate is substantially equal to 0.25 times the wavelength over asubstantial portion of the reflected bandwidth of the polariser.

By means of this embodiment it can be achieved that for a substantialportion of the reflected bandwidth the optical retardation issubstantially equal to 0.25 times the wavelength. When a singlestretched synthetic resin film is used, the optical retardation remainsthe same over the entire bandwidth. As a result, the conversion fromcircularly polarized light into linearly polarized light is not optimalover the entire bandwidth. This is a disadvantage, particularly, inbroadband polarisers.

The invention also relates to methods of manufacturing the polariser inaccordance with the invention. A first method is characterized in that amixture of chiral and nematogenic monomers, each having a differentreactivity, is provided in the form of a layer between two parallelsubstrates, after which actinic radiation is applied in accordance witha radiation profile whose intensity varies over the layer, so that themixture is polymerised to an optically active layer of polymer materialhaving a cholesteric order, whereafter, if desired, the substrates areremoved from the optically active layer.

A second method of manufacturing the inventive polariser ischaracterized according to the invention in that a mixture of chiral andnematogenic monomers, each having a different reactivity, is provided ona substrate in the form of a layer, after which actinic radiation isapplied in accordance with a radiation profile whose intensity variesover the layer, so that the mixture is polymerised to an opticallyactive layer of polymer material having a cholesteric order, whereafter,if desired, the substrate is removed from the optically active layer.

In general, it holds that the pitch of the molecular helix is governedto an important degree by the ratio between the chiral and the mesogenicmonomer in the polymer material. Owing to the difference in reactivitybetween both monomers, the capture probability of the most reactivemonomer is greater than that of the least reactive monomer. If duringthe polymerization of the mixture, which is initiated by actinicradiation, a variation in the radiation intensity is realised across theoptically active layer to be formed, the most reactive monomer ispreferentially incorporated in the polymer at the location(s) of thehighest radiation intensity. As a result, one or more concentrationgradient(s) of free monomer is (are) formed during said polymerizationprocess. This causes monomer diffusion from locations with a low monomerconcentration to locations with a high monomer concentration. Themonomers of high reactivity will diffuse to the place(s) where theradiation intensity is highest. The monomers of low reactivity, however,will diffuse to the place(s) where the radiation intensity is lowest.This leads to an increase in reactive monomer in areas of the formedpolymer material where, during the polymerization process, the radiationintensity was highest. As a result, the composition of the polymermaterial varies in the direction transverse to the polymer layer. Thiscauses a variation in the pitch of the molecular helix in the layer,which molecular helix is formed by the polymer. This variation of thepitch provides the optically active layer with a large bandwidth, thevalue of which is proportional to the value of the variation in pitch.In the first method, the mixture can be provided between the substratesby means of a pipette and vacuum techniques. In the second method, themixture can be provided on the substrate by means of a so-called"doctor's blade" or by means of screen printing. The second methodclearly is more suitable for mass production. The time-consuming fillingof narrow spaces between substrates is omitted in the second method.

It is noted that in theory the variation of the pitch of the molecularhelix can be realised on the basis of the temperature dependence of thepitch of cholesteric polymers. In this case a temperature gradient mustbe provided across the optically active layer of the mixture ofmonomers, after which photopolymerization has to take place. Due to thesmall thickness of the optically active layer, typically a fewmicrometers to several tens of micrometers, this approach leads inpractice to great problems caused by the required slope of such atemperature gradient.

It is further noted that in said inventive methods it is not absolutelynecessary to use substrates which are provided with orientation layerson the surface facing the optically active layer. Particularly in themanufacture of (very) thin optically active layers spontaneousorientation of the nematogenic groups generally occurs. However, thepresence of orientation layers during polymerization leads to animproved orientation of the optically active layer, so that the opticalproperties of the polarisers are substantially improved.

For the reactive monomers use can be made of compounds comprising areactive group on the basis of acrylates, epoxy compounds, vinyl ethersand thiolene systems, as described in, inter alia, U.S. Pat. No.5,188,760. Monomers having a different reactive group generally differin reactivity. Said reactivity is also governed by the reactionconditions under which the polymerization operation is carried out, suchas temperature, wavelength of the actinic radiation used, etc..

An interesting variant of the above-mentioned methods is characterizedaccording to the invention in that the number of reactive groups of thechiral monomer differs from that of the nematogenic monomer. When such amixture of monomers is photopolymerized a three-dimensional network isformed because at least one of the two types of monomers comprises twoor more reactive groups. As noted above, such a polymer network givesgreat strength to the optical layer, so that after thephotopolymerization process the substrates used, including anyorientation layers, can be removed from the optical layer, if desired,so that a self-supporting broadband polarization filter is obtained.Monomer mixtures in which one type of monomer comprises one reactivegroup and the other type of monomer comprises two reactive groups arepreferred.

The use of such a mixture of monomers comprising only one and the sametype of reactive group has an additional advantage. As a result of asmaller capture probability, monomers having one reactive group are lessreactive than monomers having two reactive groups. By virtue thereof anddespite the fact that one and the same type of reactive group is used, aconcentration gradient can be realised in the photopolymerizationprocess. The use of said two types of reactive monomers in a mixtureoffers the advantage of improved mixing of these monomers. Very goodresults are obtained with monomer mixtures in which one monomer has oneacrylate group and the other monomer has two acrylate groups.

Photopolymerization takes place by means of actinic radiation. This isto be understood to mean irradiation with light, preferably UV-light,X-rays, gamma rays or irradiation with high-energy particles, such aselectrons or ions. Polymerization can be carried out in several ways.For example, by means of a coherent radiation source (laser) aperiodical variation of the pitch of the molecular helix can beobtained. Constructive and destructive interference is used to form aninterference pattern in the optically active layer, so that during thepolymerization operation a periodical variation of the light intensitystraight through the layer occurs.

A simpler polymerization method is obtained if a non-coherent radiationsource is used whose wavelength is chosen to lie in the range where themaximum of the sum of the absorptions of the monomers used and thephotoinitiator is found. In this case a relatively large gradient of thelight intensity across the optically active layer can be obtainedwithout taking additional measures.

A very convenient method is characterized according to the invention inthat the mixture also comprises a dye having an absorption maximum whosewavelength corresponds substantially to the wavelength of the actinicradiation used. An important advantage of this method is that it offersgreat freedom as regards the choice of the layer thickness, thephotoinitiation system, the polymerization wavelength and thepolymerization rate. As the quantity of dye can be selected at willwithin certain limits, independent of the other components of themixture to be polymerized, the (linear) intensity gradient of theradiation across the optically active layer can be adjusted veryaccurately via the concentration of the dye. Preferably, a dye is usedwhose absorption maximum lies outside the wavelength range in which thepolarizer must operate. In this manner, undesired absorptions during theuse of the finished polarizer are precluded.

A third interesting method of manufacturing the inventive polariser ischaracterized in that a surface of an optically active layer ofpolymerised liquid crystalline material having a cholesteric order isprovided with a film of reactive monomers which cause a concentrationgradient in the layer as a result of diffusion, after which the monomersare polymerised.

Said method is based on known optically active layers of polymerisedliquid crystalline material of cholesteric order, as described in, interalia, U.S. Pat. No. 5,132,147 and U.S. Pat. No. 4,410,570. In theseknown optically active layers, the pitch of the molecular helix acrossthe layer is substantially constant. The diffusion of the monomers inthe optically active layer causes this layer to swell up slightly. Thisswelling leads to an increase of the pitch of the molecular helix. Saidincrease causes the reflection wavelength values to augment in situ.Consequently, the provision of a concentration gradient of monomeracross the thickness of the optically active layer results in avariation of the pitch of the molecular helix. As a result, a broadbandcholesteric polariser is obtained.

Due to the polymerization of the monomers the diffusion in the opticallyactive layer is stopped. Further diffusion would eventually lead to anarrow-band polariser whose reflection wavelength exceeds that of theinitial optically active layer. For the reactive monomers use can bemade of molecules comprising nematogenic groups. However, this is notnecessary. Since the diffusion causes an increase of the pitch of themolecular helix, the initial, optically active layers should have areflection which corresponds to the smallest desired wavelength.Consequently, for the manufacture of broadband polarisers which must beoperative in the entire visible range of the spectrum, use should bemade of a cholesteric layer having a reflection band in the blue regionof the visible spectrum.

An interesting variant of the third method in accordance with theinvention is characterized in that the film comprises a mixture ofmonomers having different diffusion rates in the layer. It has beenfound that the use of a single type of monomer does not always yieldsatisfactory results. This can be attributed to the formation of a sharpreflection peak. The use of two or more monomers having differentdiffusion rates causes the reflection peaks to overlap. This results ina broadband behaviour of the polariser.

Another interesting variant of the third method in accordance with theinvention is characterized in that at least a part of the monomerscomprises two or more polymerisable groups. This measure yields abroadband, optically active layer of considerable strength.Polymerization of the monomers leads to a three-dimensional network. Byvirtue thereof, the possibility of further diffusion of these monomersafter the polymerization operation is further reduced.

The invention further relates to a lighting device comprising a socketfor an electric light source, a reflector and a cholesteric polarizerwhich is preferably provided with a quarter-wave plate. This device ischaracterized in that a cholesteric polarizer in accordance with theinvention is used therein. By means of such a lighting device polarizedlight can be produced with a high efficiency. If the polarizer comprisesa quarter-wave plate, the light produced is linearly polarized. Devicesproducing linearly polarized light are particularly suitable for indoorapplications (for example office and shop lighting) and outdoorapplications (for example car and street lighting). The inventive devicecan also be used in liquid crystal displays. In linearly polarizedlight, reflections and scattering occur to a lesser degree than incircularly polarized light. In the absence of a quarter-wave plate, theinventive device produces circularly polarized light.

In the device in accordance with the invention, a discharge lamp can beused as the electric light source, in which, during operation, adischarge arc is generated, for example between two electrodes, in adischarge tube. Examples of this type of light sources are high-pressuredischarge lamps, such as high-pressure Na, Hg or metal halide lamps.Low-pressure discharge lamps, such as fluorescent lamps or low-pressureNa lamps, can also suitably be used. Alternatively, an incandescent bodyin an airtight lamp envelope can be used as the light source in thelighting device in accordance with the invention. The lamp envelope maybe evacuated or it may contain a filler gas. Said filler gas may beinert or based on a halogen gas. Halogen lamps are a good example of thelast-mentioned class of electric light sources. It is noted that thelight source may be detachably provided in the device, for example bymeans of a screw thread or bayonet fixing. It is alternatively possible,however, that the light source and the socket for said light source areinextricably interconnected.

The reflector serves to reflect the light generated by the light sourceduring operation of the inventive device in the direction of thepolarizer. The surface of the reflector facing the light source consistsof a material having a high reflection coefficient for the light of thelight source to be used. For applications with visible light, interalia, metals and alloys having a good reflectivity can suitably be used,such as preferably gold, silver and aluminium, the latter being muchcheaper. The reflector can be self-supporting but generally consists ofa metal layer which is vapour-deposited on to a housing.

The cholesteric polarizer of the inventive device preferably comprises aquarter-wave plate for converting circularly polarized light intolinearly polarized light. Said quarter-wave plate may be connected tothe cholesteric polarizer as a separate optical element. Preferably, thepolarizer and the quarter-wave plate are integrated so that they form anindividual optical element. In the latter case, said quarter-wave platemay consist of a stretched synthetic resin film which was used as thesubstrate of the optically active layer in the manufacture of thepolarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by means of exemplaryembodiments and the drawing, in which

FIG. 1a to 1c diagrammatically shows a number of embodiments ofcholesteric polarizers in accordance with the invention.

FIG. 2a and 2b shows the chemical structural formulas of two monomerswhich can be used in the manufacture of a cholesteric polarizeraccording to the invention.

FIG. 3 shows the chemical structure of a dye which can be used in themanufacture of a polarizer according to the invention.

FIG. 4 shows a reflection spectrum of a cholesteric polarizer, the pitchof the molecular helix being (a) constant and (b) varied.

FIG. 5 shows a graph in which the variation of the pitch in theoptically active layer of a polariser in accordance with FIG. 1(a) isdepicted.

FIG. 6 shows a transmission spectrum of an inventive polariser inaccordance with FIG. 1(b).

FIG. 7 shows a transmission spectrum of the inventive polariser inaccordance with FIG. 1-c, (a) before diffusion and (b) after diffusion.

FIG. 8 is a diagrammatic sectional view of a first lighting device inaccordance with the invention.

FIG. 9 is a diagrammatic sectional view of a second lighting device inaccordance with the invention.

It is noted that, for clarity, the individual components of the Figuresare not drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows a first embodiment of a polarizer in accordance with theinvention. Said polarizer comprises two flat, transparent glasssubstrates 1 and 2 which extend parallel to each other and are locatedat some distance from each other. The facing surfaces of the substratesare provided with an orientation layer 3 and 4, respectively, forexample of rubbed polyimide or sputtered SiO_(X) and the edges areprovided with a spacer 5.

An optically active layer 6 is present between both substrates. Saidlayer consists of a polymere material having a cholesteric order. Theaxis of the molecular helix of the cholesteric material extendstransversely to the layer. The pitch of the molecular helix is varied inthe layer, said pitch increasing continuously from one surface of theoptically active layer to the other surface. In the present case this isdiagrammatically shown by means of two spiral-shaped structures 7. Thethickness of the optically active layer typically ranges from 3 to 40micrometers, preferably from 5 to 25 micrometers.

The above-described embodiment of the cholesteric polarizer inaccordance with the invention was manufactured as follows. First, amixture of reactive monomers was prepared. This mixture comprises 60 wt.% of the chiral component A and 40 wt. % of the nematogenic component B.Component A comprises two reactive acrylate groups per molecule andcomponent B comprises one reactive acrylate group per molecule. Theexact structural formulas of the components A and B are shown in FIG. 2.Owing to the different number of reactive groups per molecule, bothmonomers have a different reactivity. Subsequently, 0.5 wt. % of thephotoinitiator Igacure-651 (Ciba Geigy) and 50 ppm of p-methoxyphenol(stabilizer) and a quantity of a dye are added to this mixture. Thechemical structure of this dye is shown in FIG. 3. This dye exhibits anabsorption maximum around 334 nm and an extinction coefficient of 31524l/mol.cm.

The mixture thus manufactured was then provided between two transparentsubstrates. Said substrates carried a layer of rubbed polyimide. Saidlayers are used to orient the molecular helix which forms spontaneouslyin the cholesteric mixture. To preclude the formation of disclinationsboth substrates were sheared over a small distance until a planar orderwas obtained. Subsequently, the reactive mixture was photopolymerized bymeans of UV-light for 8 minutes at room temperature. As one of the tworeactive monomers comprises two reactive groups per molecule, athree-dimensional polymer network is formed during polymerization. Dueto the strength of the optical layer thus formed, the optical layercould be detached from both substrates and used as a self-supportingcholesteric polarizer.

A number of the above-described polarizers was manufactured, thequantity of dye added being varied as well as the wavelength (λ) and theincident power (Io) of the UV-light. Subsequently, the bandwidth of thispolarizer was measured. Table 1 shows the bandwidth which corresponds toa certain quantity of dye added.

                  TABLE I                                                         ______________________________________                                                Bandwidth (nm)                                                        Dye concentration      Io = 0,058                                             (wt %)    Io = 0.62 mW/cm.sup.2                                                                      mW/cm.sup.2                                                                            Io = 0.15 mW/cm.sub.2                         ______________________________________                                        0         41           41       45                                            0.33      55           133      126                                           0.66      70           255      233                                           1.0       114          261      319                                           2.0       258          380      >400                                          4.0       316          >400                                                   6.0       362                                                                 ______________________________________                                    

In the absence of a dye the bandwidth is less than 50 nm. When a dye isused the bandwidth increases rapidly, even to values in excess of 400nm. The centre of the bandwidth is always at approximately 555 nm. Withthis bandwidth and its position in the spectrum, substantially theentire visible portion of the spectrum is covered. Consequently, such acholesteric polarizer can suitably be used as a broadband polarizer forthe entire visible light spectrum.

FIG. 4 shows a reflection spectrum of a polarizer in which the pitch is(a) constant and (b) in accordance with the first embodiment of theinvention. The polarizers were exposed to circularly polarized light.Spectrum (a) was obtained in the absence of a dye. The bandwidth of thisspectrum is therefore only approximately 45 nm. Spectrum (b) wasobtained by using a dye during the polymerization process. The bandwidthof this filter is approximately 230 nm.

                  TABLE 11                                                        ______________________________________                                        wt. % dye    bandwidth (nm)                                                   ______________________________________                                        0.00         37                                                               0.13         68                                                               0.17         82                                                               0.26         109                                                              0.41         168                                                              0.58         308                                                              0.71         326                                                              ______________________________________                                    

Table II shows the bandwidths of a number of other cholestericpolarizers in accordance with the invention. Instead of theabove-mentioned dye, the azo-dye SI-486 (Mitsui Toatsu Dyes Ltd) wasadded in certain concentrations for the manufacture of these polarizers.Said dye has an absorption maximum around 400 nm. In this case, thereactive mixture was polymerized with a UV-source (365 nm) having anincident power of 5 mW/cm² for 8 minutes. Table II also shows that thebandwidth increases with the quantity of dye.

FIG. 5 shows the change of the pitch as a function of the distance fromone surface of an 18 micrometer thick polarizer to the other surface.This Figure was obtained by means of SEM photographs taken at thefracture face of cross-sections of this polarizer. The polarizercontained 0.72 wt. % of the above-mentioned azo-dye. The reflection bandof the polarizer was approximately 350 to 800 nm. Due to the absorptionband of the dye in the range in which the polarizer can be operated,said polarizer exhibited undesired absorptions around 400 nm.

FIG. 1-b shows a second embodiment of a polariser in accordance with theinvention. Said polarizer comprises a substrate 11 carrying an opticallyactive layer 14. Said substrate 11 is composed of a first stretchedsynthetic resin film 12 of polypropylene and a second stretchedsynthetic resin film 13 of polycarbonate. The directions of stretchingof both films extend substantially transversely to each other. Thedegree of stretching of both films was selected in such a manner that,at a wavelength of 590 nm, the optical retardation of the polypropylenefilm is 518 nm, while the optical retardation of polycarbonate in theseconditions is 370 nm. Due to said choice of the conditions, thedifference in dispersion between both films causes the opticalretardation of the crossed films to be substantially equal to 0.25 timesthe wavelength over the entire wavelength range of from 400-700 nm .

A cholesteric polarizer in accordance with the second embodiment wasmanufactured as follows. The composite substrate described in thepreceding paragraph was provided, by means of a doctor's blade, with athin layer 14 (layer thickness 20 micrometers) of a mixture of reactivemonomers. A number of comparative experiments showed that this layercould alternatively be provided by screen printing. The mixturecontained both chiral and nematogenic monomers. Said two types ofmonomers had a different reactivity. The composition of the monomermixture was as follows: 58.8 wt. % of component A, 39.2 wt. % ofcomponent B, 1 wt. % of the dye in accordance with FIG. 3, 1 wt. % ofIgacure 651 (Ciba Geigy) and 100 ppm of p-methoxy phenol. After thelayer had been provided, it was polymersied by means of an UV source(365 nm) to which the layer was exposed at 100° C. and a radiationintensity ranging from 0.06 to 0.6 mW/cm² for approximately 60 minutes.After the polymerization operation, the thickness of the opticallyactive layer was approximately 18 micrometers.

FIG. 6 shows a spectrum in which the transmission T is plotted as afunction of the wavelength of a cholesteric polarizer in accordance withthe second embodiment. Line (a) denotes the transmission of p-polarizedlight, while line (b) denotes the transmission of s-polarized light. Thebandwidth of the filter was approximately 220 nanometers. It was foundthat the conversion from circularly polarized light to linearlypolarized light was optimal over the entire bandwidth.

FIG. 1c shows a third embodiment of the polarizer in accordance with theinvention. Said polarizer comprises a substrate 21 of a stretchedsynthetic resin film. In the present case, polycarbonate was used. Thedegree of stretching of the film was selected in such a manner that thebirefringence at room temperature was 0.0029. The thickness of thesubstrate was 50 micrometers. With the substrate in question an optimumconversion of circularly polarized light into linearly polarized lightwas obtained at a wavelength of approximately 580 nm. This wavelength issituated in the centre of the reflection band of the broadbandpolarizer.

An optically active layer 22 of liquid crystalline material ofcholesteric order is present on the substrate. The thickness of layer 22was 20 micrometers. Layer 22 carries a top layer 23 of polymer material.Said layer, which has a thickness of approximately 2 micrometers, servesas a protective layer of the optically active layer.

Said third embodiment of the inventive broadband polarizer wasmanufactured as follows. An optically active layer of polymerised liquidcrystalline material of cholesteric order was provided on the substrate.Techniques for applying such a layer are described in, inter alia, U.S.Pat. No. 5,132,147. In such a layer the molecular helix of thecholesteric material extends transversely to the layer. The pitch ofsaid helix is substantially constant.

A thin layer of a mixture of reactive monomers was provided on theoptically active layer. In the present case, a mixture of threedifferent monomers was used in a volume ratio of 1:1:1. For the monomersuse was made of butanediol diacrylate, octanediol acrylate andethoxylated bisphenol-A diacrylate. A quantity of 4 wt. % of aphotoinitiator was added to the mixture. The monomers diffuse in theoptically active layer at different rates. This results in the formationof a concentration gradient of monomers in the optically active layer.This causes an increase of the pitch, said increase being proportionalto the monomer concentration at the location of the optically activelayer. After a diffusion time of 10 minutes at 60° C., the reactivemonomers were polymerised to form a network by means of exposure to UVlight at a wavelength of 365 nm. Said exposure took place at 60° C. anda radiation intensity of 0.5 mW/cm² for 5 minutes. This resulted incomplete immobilisation of the monomers and stabilization of theconcentration gradient. Since the monomer mixture was not completelydiffused in the optically active layer in the polymerization operation,also a polymerised top layer (thickness 2 micrometers) was formed on theoptically active layer.

FIG. 7 shows a spectrum of the cholesteric polarizer in which thetransmission T is plotted as a function of the wavelength. The incidentlight was circularly polarized. Spectrum (a) shows the transmissioncharacteristic of the optically active layer before the diffusion of themonomers. Spectrum (b) shows the characteristic of the finishedpolarizer after the diffusion and polymerization of the monomers.Further measurements showed that the conversion of circularly polarizedlight into linearly polarized light was not optimal over the entirebandwidth of the inventive polarizer. Particularly in both edges of theband the conversion was not satisfactory. A better conversion can beobtained by using a composite substrate comprising two stretchedsynthetic resin films, the directions of stretching of both filmsextending substantially transversely to each other.

FIG. 8 is a diagrammatic sectional view of a first embodiment of thelighting device in accordance with the present invention. Said lightingdevice comprises a box-shaped housing 31, for example of syntheticresin, one inner surface of which is provided with a reflector 32 ofvapour-deposited aluminium. In the housing there are accommodated threefluorescent lamps 33 as the electric light sources. Said lamps aredetachably provided in corresponding sockets (not shown) via a threadedjoint. It is alternatively possible to use a meander-shaped fluorescenttubular lamp instead of separate fluorescent lamps. The device alsocomprises a polarizer 34 having a quarter-wave plate 35. The polarizeris constructed as a broadband polarizer, as described in the foregoingembodiments.

When the device shown in FIG. 8 is in operation, the three fluorescentlamps generate unpolarized light. A part of this light falls directly onpolarizer 34 which allows passage of one of the orthogonally linearlypolarized components, whereas the other, "compatible", component isreflected. The reflected component is (partly) converted into the"incompatible" component on reflector 32 and reflected in the directionof the polarizer which allows passage of this "incompatible" componentwhich is then converted into linearly polarized light by thequarter-wave plate. In this manner, the originally unpolarized light isconverted into linearly polarized light with a high efficiency(approximately 80%).

FIG. 9 is a diagrammatic sectional view of another embodiment of theinventive device. Said device comprises a parabolic, self-supportingreflector 41 of aluminium. Said reflector comprises a socket 42 for anelectric light source which, in this case, consists of a detachablehalogen lamp 43. The device further comprises a reflective polarizer 44having a quarter-wave plate 45. The inventive device operatesessentially in the same manner as the device of the foregoingembodiment. The embodiment shown in FIG. 9 can particularly suitably beused for car lighting or studio lighting.

We claim:
 1. A cholesteric polarizer comprising an optically activelayer of a polymer material, the polymer material having a cholestericorder and a molecular helix, the molecular helix having an axisextending transversely to the layer, wherein a pitch of the molecularhelix in the layer varies with a difference between a maximum pitch anda minimum pitch of at least 100 nm, said polymer material furthercomprising a three-dimensional network.
 2. A cholesteric polarizer asclaimed in claim 1, wherein the pitch of the molecular helix increasessubstantially continuously from a minimum value at one surface of thelayer to a maximum value at the other surface of the layer.
 3. Acholesteric polarizer as claimed in claim 2, wherein said opticallyactive layer is present on a substrate of a stretched synthetic resinfilm, the degree of stretching and the thickness of the film beingselected in such a manner that the optical retardation of the film isapproximately 0.25 times the center wavelength of the band reflectedduring operation of the polarizer.
 4. A cholesteric polarizer as claimedin claim 2, wherein said optically active layer is present on asubstrate of two stretched synthetic resin films of differentcomposition, the directions of stretching of both films extendingsubstantially transversely to each other and the degree of stretching ofboth films being selected in such a manner that, due to a difference indispersion between the films, the net retardation of the substrate issubstantially equal to 0.25 times the center wavelength over asubstantial portion of the reflected band of the polarizer.
 5. Acholesteric polarizer as claimed in claim 1, wherein said opticallyactive layer is present on a substrate of a stretched synthetic resinfilm, the degree of stretching and the thickness of the film beingselected in such a manner that the optical retardation of the film isapproximately 0.25 times the center wavelength of the center of the bandreflected during operation of the polarizer.
 6. A cholesteric polarizeras claimed in claim 1, wherein said optically active layer is present ona substrate of two stretched synthetic resin films of differentcomposition, the directions of stretching of both films extendingsubstantially transversely to each other and the degree of stretching ofboth films being selected in such a manner that, due to a difference indispersion between the films, the net retardation of the substrate issubstantially equal to 0.25 times the center wavelength over asubstantial portion of the reflected band of the polarizer.
 7. Alighting device comprising a socket for an electric light source, areflector and a cholesteric polarizer which is provided with aquarter-wave plate, wherein said cholesteric polarizer comprises anoptically active layer of a polymer material, the polymer materialhaving a cholesteric order and a molecular helix, the molecular helixhaving an axis extending transversely to the layer, wherein a pitch ofthe molecular helix in the layer varies with a difference between amaximum pitch and a minimum Pitch of at least 100 nm, said polymermaterial further comprising a three-dimensional network.
 8. The lightingdevice as claimed in claim 7, further wherein said optically activelayer is present on a substrate of a stretched synthetic resin film, thedegree of stretching and the thickness of the film being selected insuch a manner that the optical retardation of the film is approximately0.25 times the center wavelength of the band reflected during operationof the polarizer.
 9. The lighting device as claimed in claim 7, furtherwherein said optically active layer is present on a substrate of twostretched synthetic resin films of different composition, the directionsof stretching of both films extending substantially transversely to eachother and the degree of stretching of both films being selected in sucha manner that, due to a difference in dispersion between the films, thenet retardation of the substrate is substantially equal to 0.25 timesthe center wavelength over a substantial portion of the reflected bandof the polarizer.
 10. A lighting device comprising a socket for anelectric light source, a reflector, and a cholesteric polarizer which isprovided with a quarter-wave plate, characterized in that saidcholesteric polarizer comprises an optically active layer of a polymermaterial, the polymer material having a cholesteric order and amolecular helix, the molecular helix having an axis extendingtransversely to the layer, wherein a pitch of the molecular helix in thelayer varies with a difference between a maximum pitch and a minimumpitch of at least 100 nm, further characterized in that the pitch of themolecular helix increases substantially continuously from a minimumvalue at one surface of the layer to a maximum value at the othersurface of the layer, further wherein said polymer material furthercomprises a three-dimensional network.
 11. The lighting device asclaimed in claim 10, further wherein said optically active layer ispresent on a substrate of a stretched synthetic resin film, the degreeof stretching and the thickness of the film being selected in such amanner that the optical retardation of the film is approximately 0.25times the center wavelength of the band reflected during operation ofthe polarizer.
 12. The lighting device as claimed in claim 10, furtherwherein said optically active layer is present on a substrate of twostretched synthetic resin films of different composition, the directionsof stretching of both films extending substantially transversely to eachother and the degree of stretching of both films being selected in sucha manner that, due to a difference in dispersion between the films, thenet retardation of the substrate is substantially equal to 0.25 timesthe center wavelength over a substantial portion of the reflected bandof the polarizer.